Merge tag '4.0.0' into develop

4.0.0
Remove WordPress core as submodule
Add WordPress core as Composer package repository type. Set custom install path.
Amend WordPress composer.json WordPress def.

Remove custom "getdave" vendor. Not required and appears to trigger
minimum stability warning.
Update composer.lock and ensure wp dir is ignored
Merge branch 'release/5.0.0'